In most thermal power stations electrical energy is generated by first producing superheated steam by means of burning fossil fuels in boiler installations. The steam is expanded in steam turbines and, in doing so, converted into mechanical energy. The steam turbines are coupled with electric generators so that this mechanical energy is converted into electrical energy. This is effected with an efficiency of well over 90%. On the other hand, the efficiency of the conversion of the energy chemically bonded in the utilized fuel into mechanical energy is quite modest, as the turbine efficiency is at most approximately 37% even in large turbines, and losses in the heating boiler must also be taken into account.
Therefore, in many cases only roughly 35% of the heat released during combustion could previously be effectively used for generating electricity, while roughly 65% was lost as exhaust heat or could only be used purely for heating purposes.
More recently, a considerable increase in mechanical or electrical efficiency was achieved by employing a combination of gas turbines and steam turbines for converting the thermal energy into mechanical energy. The hot combustion gases are first expanded in gas turbines and the heat of the exhaust gas of these gas turbines is used for generating the steam for the steam turbines. Other possibilities for improvement consist in guiding the expanded steam flowing out of a steam turbine back into the combustion chamber of the gas turbine connected upstream, thus generating a greater volume flow for driving the gas turbine. These steps have made it possible to raise the efficiency of the conversion of thermal energy into mechanical energy in larger plants (over 50 MW) in the order of magnitude of approximately 48 to 50%.
A process and an installation for generating mechanical energy from gaseous fuels is known from the European Patent 0 318 122 A2, in which the mechanical energy which can be used, for example, to generate current is delivered solely by means of a gas turbine, rather than partially by means of a steam turbine. This gas turbine, which is provided particularly for an output range of 50 to 3000 KW, achieves an efficiency of approximately 42% with respect to the utilized thermal energy (net calorific value). To this end, combustion air is first compressed in a compressor. The compressed combustion air is then heated in an exhaust gas heat exchanger, partially expanded via a first gas turbine which only drives the compressor, and subsequently fed to a combustion chamber in which fuel is burned with this combustion air.
The hot exhaust gas formed during combustion drives a second gas turbine which supplies the actual usable mechanical energy. The still hot exhaust gas flowing out of the second gas turbine is used for operating the exhaust gas heat exchanger for heating the compressed combustion air.
In the German Patent 40 03 210.8, which was not published beforehand, the Applicants already suggested a process for generating mechanical energy which can be converted into electrical energy by means of an electric generator. This process provides that a starting fuel based on hydrocarbon compounds is first converted in a steam reformation into a hydrogen-rich gas of superior value from an energy standpoint before this hydrogen-rich gas is burned in one or more combustion chambers. The combustion is effected by means of a compressed oxygen-containing gas (e.g. compressed air). The generated hot combustion gas is expanded in a gas turbine generating the externally deliverable or output mechanical energy, is correspondingly cooled off and then used for indirect heating of the steam reformer. The combustion gas which is further cooled in the steam reformer is then used for heating the compressed combustion air in a further indirect heat exchange. The compressed combustion air accordingly obtains so much energy that it can be partially expanded in a gas turbine before being used for the combustion and thus supplies the required drive energy for generating compressed air. In another variant of this process, the compressed combustion air which is heated by the indirect heat exchange is first guided into a combustion chamber and is there burned with a portion of the hydrogen-rich gas so that a still hotter gas is available for expansion in the gas turbine.
This process makes it possible to increase the efficiency of the conversion of the energy (net calorific value H.sub.u) contained in a conventional fuel (e.g. natural gas or biogas) into mechanical energy at a reasonable cost in small plants (up to approximately 3 MW output) by at least 50% and in larger plants by at least 55%.
As a rule, it is provided in such processes ultimately to convert the generated mechanical energy into electric current. This is because energy can be most easily transported to the desired location with an energy requirement in this form and can be converted back into other forms of energy (e.g. mechanical or thermal) with high efficiency in a comparably simple manner. On the other hand, the increasing demand for substantial reductions in CO.sub.2 and other pollutants (particularly NO.sub.x, SO.sub.x) formed in the conversion of fuels into electric current or mechanical energy must be taken into account. With respect to CO.sub.2, this demand can be met without incurring the costs for separating CO.sub.2 from the occurring exhaust gases only if the energy chemically bonded in the utilized fuel is converted in a considerably more efficient manner than was previously the case. Thus there is a need for a further increase in the efficiency of energy conversion not only for purely economic reasons but also for purposes of environmental protection.